At its narrowest, the case addresses the specific (though important) question of whether the power to fund prophylactic medicine for HIV lies with local authorities or with the NHS Commissioning Board (NHS England).

More generally, it serves as an unflattering critique of the legislation which underpins the allocation of roles and responsibilities within the health service.

And, at its widest, it adds usefully to the case law on how to understand the vires of a public authority when it lies within a badly-drafted, and hard to interpret, statutory regime.

If a public body adopts a policy about how it will exercise one of its functions, it must follow it.

This principle has been developed over the last 15 years in a series of cases brought against the Secretary of State for the Home Department. Initially it was regarded as an outworking of the doctrine of legitimate expectation (Saadi at [7]). More recently it has been treated as a freestanding ground of judicial review which is ‘a requirement of good administration‘ (Nadarajah at [68]) and ‘a basic public law right‘ (Lumba at [58]).

The full extent to which the duty has now been cut loose from its traditional moorings in legitimate expectation became evident last year in the Supreme Court case of Mandalia. In that case the claimant successfully relied on the Home Office’s failure to follow an internal policy – a ‘process instruction’ to civil servants – of which he neither had, nor could have had, any knowledge at the time when it should have been applied. Following Mandalia, even an unpublished policy is now binding.

The duty is qualified by another basic public law principle, that policies should not fetter discretion. If the circumstances of an individual case provide a ‘good reason‘ for doing so (Lumba at [26]) a public body may, and sometimes must, depart from its own policy.

Subject to this qualification, the requirement to follow existing policies has developed into an important obligation on public bodies. However, two recent cases expose some of the limits of reliance on policies as a ground of public law challenge.

Each year, at least 1000 young women, but perhaps as many as twice that number, travel from Northern Ireland to another part of the United Kingdom in search of an abortion. They do so to avoid the criminal liability that (in most cases) still attaches to terminations of pregnancy in Northern Ireland, and to rely instead on the more liberal legal regime prevailing in England, Scotland and Wales.

Should they be entitled to obtain abortions free on the National Health Service?

The answer to this question, which was considered by the Court of Appeal inA (a child) v Secretary of State for Health, reveals the full extent to which, under the UK’s devolution settlement, the disaggregation of the NHS into four discrete health services – one for each of the four constituent territories* of the United Kingdom – is now legally entrenched. By implication, it also asks important questions about the effect of future devolution within England.